This application is based on Application No. 2000-223896, filed in Japan on Jul. 25, 2000, the contents of which are hereby incorporated by reference.
This invention relates to an ac generator and, more particularly, to a vehicular alternating current generator driven by an engine.
FIG. 12 is a sectional side view showing one example of a conventional vehicular ac generator. As shown in FIG. 12, the generator comprises a case 3 composed of a front bracket 1 and a rear bracket made of aluminum, a shaft disposed within the case 3 and having a pulley 4 secured to one end portion, a, Randell-type rotor 7 secured to the shaft 6, a fan 5 fixed to the opposite ends of the rotor 7 and a stator 8 secured to an inner surface of the case 3.
The generator further comprises a slip ring 9 attached to the other end of the shaft 6 for supplying an electric current to the rotor 7, a pair of brushes 10 sliding on the slip ring 9, a brush holder 11 housing the brushes 10 therein, a rectifier 12 electrically connected to the stator 8 for rectifying an alternating current generated in the stator 8 into a direct current, a heat sink 19 fitted over the brush holder 11 and a regulator 20 attached to the heat sink 19 and regulating the magnitude of the ac voltage generated in the stator 8. The front bracket 1 and the rear bracket 2 each has an exhaust window 17 which serves as a ventilation port for a cooling wind.
The rotor 7 comprises a cylindrical rotor coil 13 through which an electric current flows for generating magnetic fluxes and a pole core 14 disposed to cover the rotor coil 13 for generating a magnetic core.
The stator 8 comprises a stator core 15 and a stator coil 16 wound on the stator core 15 and generating an alternating current due to the change in magnetic fluxes from the rotor coil 13 upon the rotation of the rotor 7.
The pole core 14 comprises a pole core member 22 including a pair of first pole core member 21 and a second pole core member 22 meshing with each other. The pole core member 21 and the pole core member 22 are usually made of iron and comprises cylindrical portions 21e and 22e to which the rotor coil 13 is wound and base portions 21k and 22k from which the cylindrical portions 21e and 22e are projected. Disposed respectively at the outer edges of the base portions 21k and 22k and between the outer circumference of the rotor coil 13 and the inner circumference are plurality of paw-like magnetic poles 23 and 24 meshing with each other.
The pawl-like magnetic poles 23 and 24 have a large thickness and width at the base 21k and 22k and smaller thickness and width toward the tip end. The inner circumferential surfaces 23a and 24a of the pawl-like magnetic poles 23 and 24 have thinner thickness at the tip portion and the outer circumferential surfaces 23b and 24b are curved in an arc along the inner circumferential surface of the stator 8. The pawl-like magnetic poles 23 and 24 have two trapezoidal side surfaces 23c and 24c in relation to the circumferential direction of the rotor 7. Since the respective pawl-like magnetic poles 23 and 24 are placed in an alternatingly meshing relationship with their tip opposing to each other, the inclined faces of the inner circumferential surfaces 23a and 25a of the pawl-like magnetic poles 23 and 24 are arranged in a circumferential raw in a alternating relationship. Also, the side surfaces 23c and 24c of the pawl-like magnetic poles 23 and 24 are inclined toward the centers of the pawl-like magnetic poles 23 and 24 so that they become gradually thinner at the tip portion than at the root portion.
Secured between the adjacent pawl-like magnetic poles 23 and 24 are permanent magnets 30A of a substantially rectangular parallelepiped configuration so magnetized that reduces the leakage of the magnetic flux between the opposing side surfaces 23c and 24c. 
The operation will now be described. When an electric current is supplied to the rotor coil 13 from the unillustrated battery through the brush 10 and the slip ring 9, a magnetic flux is generated to magnetize the pawl-like magnetic pole 23 of the first pole core member 21 into the N pole and the pawl-like magnetic pole 24 of the second pole core member 22 into the S pole. On the other hand, the engine rotates the pulley 4 and the shaft 6 rotates the rotor 7, so that an alternating electromotive force is generated at the stator coil 16. This alternating electromotive force is regulated into a direct current through the rectifier 12 and is regulated at its magnitude by the regulator 20, thereby to charge the unillustrated battery.
The magnet 30A of a substantially rectangular parallelepiped configuration secured between the pawl-like magnetic poles 23 and 24 is a plastic magnet. As for the magnet material, a ferrite magnet is advantageous from the viewpoint of cost, but this material is seldom used because of the mechanical brittleness, the low magnetizable residual magnetic flux density and the heat sensitive properties. Therefore, as for the magnet material, because of the advantages of the large degree of freedom in the magnet configuration and the high residual magnetic flux density, plastic magnet is often utilized. As for the plastic magnet, neodymium-iron-born group (Ndxe2x80x94Fexe2x80x94Coxe2x80x94B bond magnet) and Samarium-iron group (Smxe2x80x94Fexe2x80x94N bond magnet) have been used.
The temperature coefficient of the residual magnetic flux density Br of the Ndxe2x80x94Fexe2x80x94Coxe2x80x94B bond magnet is xe2x88x920.1%/K (negative temperature coefficient) and the temperature coefficient of the residual magnetic flux density of the Smxe2x80x94Fexe2x80x94N bond magnet is xe2x88x920.07%/K (negative temperature coefficient), so that the magnet effect is reduced to lower the generator output when the ac generator is at an elevated temperature condition.
Generally, a typical magnet exhibits the phenomenon of the nonreversible demagnetizing, in which phenomenon the magnetic flux (magnetic force) does not recover to the initial property value after the magnet heated to an elevated temperature is returned to the room temperature, and such the rate of change is referred to as the non-reversible demagnetizing factor. Here, the non-reversible demagnetizing factor where the magnet is heated to 373K and the heating time is 2 hours is referred to as 2-hour non-reversible demagnetizing factor, and the one that the heating time is 300 hours is referred to as 300-hour non-reversible demagnetizing factor, then the 2-hour non-reversible demagnetizing factor (373Kxc3x972 hr) of the Ndxe2x80x94Fexe2x80x94Coxe2x80x94B bond magnet is xe2x88x924.4% and the 300-hour non-reversible demagnetizing factor (373Kxc3x97300 hr) is xe2x88x925.4%. The 2-hour non-reversible demagnetizing factor (373Kxc3x972 hr) of the Smxe2x80x94Fexe2x80x94N bond magnet is xe2x88x924.0% and the 300-hour non-reversible demagnetizing factor (373Kxc3x97300 hr) is xe2x88x925.3%. Therefore, when the ac generator is continuously used at an elevated temperature, the magnetic property of the magnet is deteriorated and the power of the ac generator is decreased as compared to that at the initial value.
On the other hand, the oxygen content of the Ndxe2x80x94Fexe2x80x94Coxe2x80x94B magnetic powder after heating (373Kxc3x97300 hr) is 0.8 wt % and the oxygen content of Smxe2x80x94Fexe2x80x94N magnetic powder after heating (373Kxc3x97300 hr) is 0.4 wt %. The larger the oxygen content, the more easily rust is generated on the magnetic powder and the magnetic poles due to the ingress of moisture or the like. When the rust is generated, the magnet strength and the bonding strength between the magnetic poles and the magnet is decreased, significantly reducing the rotor strength at a high speed rotation. Particularly, the oxygen content of the Ndxe2x80x94Fexe2x80x94Coxe2x80x94B bond magnet is as high as twice of that of the Smxe2x80x94Fexe2x80x94N bond magnet and is inferior in the oxygen-resistance, so that the surface treatment such as an epoxy coating or plating is necessary and costly.
Accordingly, an object of the present invention is to provide an ac generator that is inexpensive and the ac generator output power does not decrease even during the high temperature operation and that the rotor strength at a high speed rotation is sufficiently large.
The present invention resides in an ac generator comprising a stator and a rotor, and the stator is disposed within a bracket having an exhaust window and generating a three-phase ac current by a rotating field of the rotor. The rotor comprises a rotor coil for generating a magnetic flux, a pole core composed of first and second pole core members disposed so as to cover the rotor coil and having pawl-shaped magnetic poles projecting in staggered and alternating relationship, a plurality of permanent magnets disposed on both side surfaces of the pawl-shaped magnetic poles for reducing the leakage of the magnetic flux between the side surfaces of the adjacent pawl-shaped magnetic poles, and a fan mounted to each of opposite axial ends of the rotor for cooling a heat-generating member heated due to a generator output current. The permanent magnets are permanent magnets of samarium-iron alloy containing Ti and B.
The permanent magnets may be plastic magnets made of magnet powder bonded together by a resin, bonded magnets of Sm8.2xe2x80x94Fe75.6xe2x80x94Ti2.3xe2x80x94B0.9xe2x80x94N13, which may be supported by corrosion-resistive holding members surrounding the magnet or which may be independently attached to each of the magnetic poles of the first and second pole core members.
At least one portion of the side opposing to the pawl-shaped magnetic pole side surfaces of the permanent magnet may be resin-coated.
The first and second pole core members may have on their outer circumferences restricting means for restricting the displacement of the magnetic pole in the radial direction due to a centrifugal force during the rotor rotation, and the restricting means may be disposed in the vicinity of the tips of the magnetic poles of the first and second pole core members to restrict the displacement of the pole tips. The restricting means may be corrosion-resistant annular member s circumferentially extending over the entire circumference of the rotor.